Parking brake light indicator

[BHaas35]

‘95 M35A3 - Just replaced my parking brake switch. By no means a cheap part... however, the light comes on when I release the lever to the down postion. Seems counterintuitive to have the brake light on when released and off when engaged. I cannot remember what it did prior to me replacing. Can one confirm if this is correct? Thanks in advanced.

 

[HDN]

The parking brake light is supposed to be off when the parking brake is released. If it's on or flashing and the parking brake is released, it means there's a problem with the brake system, probably a leaky hydraulic circuit. In your case, maybe you have the wires reversed somewhere?

 

[BHaas35]

Appreciate the help HDN! Direct swapped the new sensor with the old: one plug into power that ran under truck and ground to bolt near brake. This morning, I unbolted the sensor and turned the truck on. I pressed the pressure plate to complete the circuit with the button and the light extinguishes. The light comes on only when I released the sensor and not completing the circuit. This is definitely backwards since when engaging the brake it pushes the bar up and presses against the switch. Could I have received a switch that was wired incorrectly? Reason I have to ask is I cannot find my old switch anywhere in the hangar at this time...still looking.

 

[HDN]

Are you sure you got the right switch? I'm not sure if similar parking brake levers on other trucks, such as the HMMWV, have a switch that's backwards from the M44 family of vehicles.

I'm assuming it's this part here which should be the same as those USAF deuces that were built with split-circuit brakes:

 

[BHaas35]

Affirm. That's the switch. P/N: 11669094-1 purchased from Eastern. I am stumped then. I have to keep digging to find my old switch and see what the differences are if any...this replacement almost looks fool proof...hold my beer

 

[HDN]

I agree, it does look foolproof. Obvious grounding eye is obvious, for one

I've never messed with the parking brake light switch and I hope I don't have to for awhile! What happens when you press the brake pedal while the light is on? I'm wondering if something else is backwards with the wiring somewhere. Has your truck always had this problem?

 

[BHaas35]

Unfortunately, I can't remember what the truck did prior to me replacing. I did just purchased her a few weeks ago. When pressing the brake pedal, nothing changes. I found the old switch. Took my multimeter to it and the new switch. Both switches act the same. They complete the circuit only when not touching the button and thereby, activating the light. Switch is only pressed when the lever is up (to me pressing the connector to the button should complete the ground and turn the light on...but no it's backwards???). I went under the dash and see the brake light is fool proof as well with a male/female connector and both plugged in correctly. I traced the wiring diagram but only odd thing I see is it's also connected to the differential switch. I have no idea what I am missing here....maybe something mechanical with the lever?

 

[HDN]

It sounds like an issue with either the switch itself or something on the parking brake lever pressing on the switch in a way that it works backwards, as in making contact when the lever is released. Other than checking the parking brake lever for something out of place that would be making the switch work backwards, I'm out of ideas and would suggest giving Eastern Surplus a call.

Is there a reason why you replaced the switch in the first place? It sounds like the old one still works.

 

[BHaas35]

Spent a few hours today looking at the lever and switch. Can’t install backwards, upside down. Reversed, no special bar to place in between. All makes mechanical sense. Lever pushes the bar up and pushes the button in. Only thing not making sense is why the light is turning off with the button depressed. This has me scratching my head.

I had all the interior taken out and redid the floor with rhino liner and reupholstered the seats. I noticed there was fraying on the wires and bare metal was showing. In stead of taping it up I decided to replace it.

I have not really driven her around since I’m waiting on the seats but maybe wondering if it has to do with building the hydraulic pressure or something before the light extinguishes. No idea but I suppose I will ignore the light for now and just triple check to put the lever down prior to driving.